Cars typically have between 1-4 speed sensors that work together to monitor the vehicle’s speed and provide information to various systems. The exact number and type of speed sensors can vary depending on the car’s make, model, and features.
What is a Speed Sensor?
A speed sensor is a device that measures the rotation speed of a wheel or drive shaft and converts it into an electrical signal. This signal is then sent to the vehicle’s engine control unit (ECU) or transmission control unit (TCU).
Speed sensors allow different systems in the vehicle to determine and respond to the current driving speed. For example, they provide data for the speedometer, allow the anti-lock braking system (ABS) to prevent wheel lock-up during hard braking, and enable cruise control systems to maintain a constant speed.
Types of Speed Sensors in Vehicles
There are a few common types of speed sensors used in automobiles:
- Magnetic speed sensor – Uses a magnetic coil and sensor to detect pulses from a spinning tone ring installed on a wheel hub or transmission shaft. This type is common for measuring wheel speed.
- Hall effect sensor – Generates a voltage based on magnetic field changes from a tone ring. Often used for transmission or driveshaft speed.
- Optical speed sensor – Uses an LED light source pointed at a disc with holes that spins with a driveshaft. The pulses of light passing through the holes measures speed.
- GPS speed sensor – Uses GPS data to directly determine the vehicle’s speed. Though not as common as other options.
Wheel Speed Sensors
Virtually all modern vehicles have wheel speed sensors to monitor the rotational speed of each wheel individually. Usually one sensor per wheel is standard.
Wheel speed sensors are typically magnetic pick-up type sensors mounted in the wheel hub or brake assembly aimed at a toothed tone ring that rotates with the wheel. They provide the most accurate direct measurement of how fast each wheel is turning.
Data from the wheel speed sensors is used by the ABS, traction control system (TCS), and stability control system like electronic stability program (ESP). The ECU compares signals from each wheel speed sensor to determine if any wheel is rotating at an unexpected speed, indicating slipping or lock-up.
Wheel speed sensors may also communicate data to the engine and transmission control units to adjust things like fuel injection, transmission shift points, and torque distribution in all-wheel drive systems.
Front Wheel Speed Sensors
Front wheel speed sensors monitor the front two wheels. These are used to detect speed differences between the sides that may indicate understeer or oversteer sliding in a turn. The stability control system uses this info to brake individual wheels and counteract the slide.
Rear Wheel Speed Sensors
Rear wheel speed sensors measure the back two wheels. Along with monitoring for slip compared to the front wheels, the rear sensors help detect locking up during braking. ABS uses the data to release a locked rear wheel.
Driveshaft Speed Sensor
Most rear-wheel drive vehicles also have a driveshaft speed sensor mounted on the rear differential. This measures the speed of the driveshaft coming out of the transmission to the rear wheels.
Driveshaft speed sensors are typically hall effect or optical sensors aimed at ring gears on the driveshaft. They provide supplemental speed measurement and redundancy in case a wheel speed sensor fails.
The ECU may compare driveshaft speed to rear wheel speed to double-check for slippage. The driveshaft sensor also provides an average speed signal for the vehicle speedometer in place of front wheel sensors.
Transmission Speed Sensor
An input shaft speed (ISS) sensor and/or output shaft speed (OSS) sensor are used on vehicles with automatic transmissions. These measure rotational speeds within the transmission.
The input shaft speed sensor monitors engine rpm going into the transmission. The output shaft speed sensor measures rpm coming out of the transmission before the driveshaft.
These transmission speed sensors are needed for the computer to control automatic shifting. They detect gear shifts and provide feedback on the current gear ratio for the transmission controller.
GPS Vehicle Speed Sensor
Some newer vehicles also have GPS capability that can provide direct vehicle speed measurements. While not yet common, GPS speed sensors are used for supplemental speed data on some models.
A GPS speed signal has the advantage of determining speed over ground rather than just wheel rotations. This allows it to account for slipping or changes in effective wheel diameter from factors like tire wear.
But GPS sensors only work when satellite signals are available and can be less precise than wheel speed sensors. So they are not relied on alone but used to enhance info from traditional sensors.
How Many Speed Sensors Do Cars Have?
Looking at common sensor configurations, most modern cars have:
- 4 wheel speed sensors (one at each wheel)
- 1 driveshaft speed sensor on rear-wheel drive vehicles
- 1 or 2 transmission shaft speed sensors on vehicles with automatic transmissions
- Some additional cars also have a supplemental GPS speed sensor
So in total, a typical car will have between 1 and 4 speed sensors, with luxury cars sometimes having up to 5 or 6 when accounting for redundancies.
One Speed Sensor
The minimum most cars have is a single transmission speed sensor to monitor shifting on automatic transmission vehicles. Cars with manual transmissions may not have any speed sensors at all, relying on the wheel speed calculated by the ABS system.
Two Speed Sensors
An automatic transmission vehicle will generally have two speed sensors – one input shaft and one output shaft sensor within the transmission.
Three Speed Sensors
Many rear-wheel drive cars with manual transmissions will have three speed sensors – one at each of the two rear wheels and one on the driveshaft.
Four Speed Sensors
Four sensors for each of the four wheels is the typical configuration on many vehicles. Having individual wheel speed sensors aids the ABS and stability control systems.
Five or More Speed Sensors
Some high-end vehicles go beyond the basics to provide redundancy and supplemental speed data. For example, having wheel speed, driveshaft, transmission, and GPS sensors combines data from multiple sources.
Speed Sensor Locations
Speed sensors are mounted in strategic locations to monitor wheel and drivetrain components. Here are some of the common speed sensor positions on a car:
- Wheel hubs – Wheel speed sensors screwed into the wheel hub or brake assembly behind each wheel.
- Transmission – Input and output shaft speed sensors installed into the transmission housing.
- Rear differential – Driveshaft speed sensor mounted on rear differential aimed at a ring gear on the driveshaft.
- ABS control module – Some wheel speed sensors send signals directly to the ABS module instead of the engine computer.
- Steering column – Rarely, some GPS sensors are located in the steering column housing.
Technicians use the expected sensor locations when diagnosing speed sensor issues. Knowing where they are makes it easier to inspect sensors and wiring for damage that could cause failures.
Speed Sensor Diagnostics and Failures
Since so many essential vehicle systems rely on accurate speed measurements, a faulty speed sensor can cause issues like:
- ABS and stability/traction control malfunctions
- Transmission shifting problems
- Incorrect speedometer readings
- Check engine light illumination
Some common speed sensor failures include:
- Sensor resistance goes out of range due to factors like excessive air gap from debris.
- Tone ring damaged from rocks or debris causes an irregular signal.
- Wiring damaged from rubbing or corrosion causes open or short circuits.
- Sensor or wiring failures from heat damage.
- Internal component failures from normal wear over high mileage use.
Diagnosing bad speed sensors starts with scanning for diagnostic trouble codes. This points to sensor faults and which sensor is affected. Then mechanics inspect wiring condition and sensor air gaps. Replacement of the sensor is needed if faults are found.
Speed Sensor Replacement
Here are some general speed sensor replacement guidelines:
- Wheel speed sensors can typically be replaced individually. But other sensors may require transmission or differential removal.
- Remove any components to access the sensor and disconnect wiring.
- Inspect tone ring/gear teeth for damage, repair if needed.
- Install new speed sensor with proper air gap per specifications.
- Clear any codes and road test to confirm normal operation.
Cost for just the replacement sensor ranges from about $25 up to $75 or more depending on the sensor type and vehicle. Labor can add $150 or more for difficult access sensors.
Conclusion
Modern vehicles rely on input from speed sensors for essential functions and to optimize performance. While configurations vary, most cars have between 1 to 4 sensors monitoring individual wheel speed, driveshaft speed, and transmission speed.
Knowing the different types of speed sensors and their locations helps when diagnosing issues. A fault in any sensor can lead to problems for systems like ABS, transmission shifting, and stability control. Regular inspection and replacement of damaged speed sensors is needed to keep all systems running properly.